Known additive components suitably employed for cutting tools are MgO and ZrO.sub.2 as disclosed in JP-B-60-16388 and 60-20346 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Al.sub.2 O.sub.3, WC, Y.sub.2 O.sub.3, etc., are also disclosed in JP-A-56-73670 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, though not being limited to cutting tools, addition of magnesia-alumina spinel and partially stabilized ZrO.sub.2 is disclosed in JP-A-60-77174 to improve mechanical properties.
The above-mentioned conventional techniques are useful for improving strength and toughness; in addition to such improvements, however, further reduction of wear is required to prolong a tool life.